This section provides background information related to the present disclosure which is not necessarily prior art.
Conventional electronic control systems for control of gas flow to a gas-fired appliance include a gas valve member that is moved by a magnetic field generated by a coil to vary gas flow rate. For this purpose, a control unit controls actuation of one or more gas valve relays through which power is supplied to the coil. For example, in a typical two-stage heating furnace, at least two gas valve relays (Stage I and Stage II) are provided in the control unit, also referred to as furnace control unit. When there is a call for heat at a particular stage, for example, at Stage II, both the gas valve relays gets ON, and a 24VAC signal is applied to an ON/OFF type gas valve so as to open the gas valve to Stage II.
FIG. 1 shows a typical electronic controlled gas valve system 100 for control of a two-stage gas valve, hereinafter interchangeably referred to as “gas valve”. The gas valve (not shown here) can be a mechanical or an electro-mechanical valve driven by a gas valve control unit 104. The gas valve control unit 104 is, in turn, controlled by a control unit 102, which is electronically coupled to the gas valve control unit 104.
The control unit 102 receives a 24VAC input from an AC power source 106, and through gas valve relays R1 and R2, sends 24VAC signals to the gas valve control unit 104 over wires w1 and w2. The relays R1 and R2 may be part of a consolidated relay unit 105 having one or more relays integrated into it as per the requirement. Typically, the relays R1 and R2 are coupled to the AC power source through at least one rectifier circuit (not shown here), typically a half-bridge rectifier, such that the rectifier circuit provides a half-wave rectified supply to the relay coil(s) for switching them ON/OFF. Also, an un-rectified 24VAC supply is provided to relay contacts so that the relays R1 and R2 send 24VAC signals to the gas valve control unit 104.
Further, the gas valve control unit 104 is shown to include two hardware circuits 108 and 110. The hardware circuits 108 and 110 are configured to process the signals received from the respective relays R1 and R2 of the control unit 102. Outputs from the respective hardware circuits 108 and 110 are sent to a controller device 112, such as a microcontroller, which is configured to control energizing/de-energizing of a coil 124 coupled to the gas valve control unit 104. In operation, the hardware circuits 108 and 110 detect the presence of signals from the relays R1 and R2 for the controller device 112. When there is a call for heat, for example, at Stage II, both the relays R1 and R2 send 24VAC signals to the gas valve control unit 104. The hardware circuits 108 and 110 make the 24VAC signals from the two relays R1 and R2 out of phase. The controller device 112 checks whether the received signals are out of phase with each other before it opens the mechanical gas valve for Stage II.
The microcontroller 112 is powered by a microcontroller power supply 114, which is in series connection to a power supply circuit 116 of the gas valve control unit 104. The power supply circuit 116 gets power from the 24VAC power source 106 through relay R1. When there is a call for heat, relay R1 gets energized and provides 24VAC power to the gas valve control unit 104.
Apart from receiving the signals from the hardware circuits 108 and 110, the controller device 112 may also receive other input signals, for example, input from a gas selection unit 118, which could suggest an alternative fuel source and/or inputs from a first and a second gas valve feedback units 120 and 122, which could be photo-interrupters used for determining current position of the two-stage gas valve. The first and second feedback units 120 and 122 may provide analog or digital signals to indicate the current position of the gas valve.
The controller device 112 processes the inputs to provide output signals for regulating power supplied to the coil 124. For this purpose, the coil 124 is coupled to the controller device 112 via two voltage-controlled switching devices 126 and 128, which may include field-effect transistors (FETs) or similar devices. The microcontroller provides a digital voltage signal, such as a “High” signal or “Low” signal, to one of the switching devices (e.g., switching device 126), and a pulse width modulated (PWM) duty cycle signal to the other switching device (e.g., switching device 128). Based on the signals received, the switching devices 126 and 128 control magnetization of the coil 124, which consequently impacts a current position of the two-stage gas valve so as to control the gas flow through the gas valve to the gas-fired appliance.
In the conventional control systems for controlling the movement of the gas valve as discussed above, at least two gas valve relays are required. The cost of implementing two gas valve relays is relatively high. In addition, there are also space constraints.
Moreover, if the Stage I or the Stage II gas valve relay gets stuck or the associated hardware fails due to some reason, the 24 VAC signal nevertheless is constantly applied to the gas valve control unit and the associated coil and/or the gas valve may remain continuously energized or open.
In conventional control unit designs, such a state is displayed as an “error” condition and the control unit operates towards turning the gas valve relays OFF. But since one or more of the gas valve relays has already failed, the control unit does not have the actual control of the operation of the gas valve, and the gas may continue to leak unchecked from the gas valve.
Further, in case of a multiple stage gas valve control (control of more than two stages), the conventional control units communicate with the gas valve through communication protocols, such as Climate Talk®, to operate the valve in a particular stage. Implementation of a communication network may be a relatively cost intensive approach, which may increase the overall cost of the control system.
There seems to be certain solutions available in the market for controlling the operation of electronically controlled gas valves as discussed above. For example, U.S. Pat. No. 4,832,594 provides a control system with time redundancy, wherein the integrated controller is designed to have 3 timers. Two of the timers form the lower and upper time limits, during which the third timer must enable the gas valve relay. If any of the timers is faulty, the gas valve relay will not work.
Further, US20100075264 discloses a redundant ignition control circuit that includes a main microprocessor and a supervisory microprocessor, which communicate with each other through PWM signals. Each microprocessor uses a PWM signal to activate the relay under its control, based on the current mode of operation of the gas valve. The relays may be replaced with MOSFETs or BJTs in different implementations. But these aforesaid solutions have limitations and are vulnerable to safety lapses on the part of the technician. Moreover, there is a need for solutions that are cost efficient, simple to implement, and have easy back or reverse compatibility.